The free aqueous phase was obtained from individual samples of dental plaque and the plaque matrix was then eluted at low pH in an attempt to remove bound protein. The fluid and the low pH phases were assayed for secretory immunoglobulin A (SIgA), IgG, IgM, the third component of complement (C'3), lysozyme, lactoperoxidase and lactoferrin. The presence of these specific and non-specific immune factors in the free and bound phases suggest they are important in host defense at the plaque-enamel interface.